There aredifferent possible combination of the four nitrogen bases taken three at a time

Explanation:

By controlling the synthesis of enzymes, DNA controls the functioning of the cell. Of the four different bases taken three at a time, there are 43, or 64, possible combinations. The meaning of each of these combinations, or codons, is known. Most of them represent one of the 20 particular amino acids found in protein.

stratosphere: as the altitude increases, the air temperature increases. the stratosphere has a layer of ozone, called the ozone layer. this layer absorbs most of the ultraviolet radiation from sunlight. this results in the stratosphere being warmer.

it has one origin of replication, but it proceeds in two directions - d)